Storage battery

ABSTRACT

A storage battery is provided in which an expand grid is improved with respect to the widths of grid wires  1   b , the sectional areas of nodes  1   e , and the shapes of meshes  1   c , whereby the productivity of the expand grid can be enhanced and the life performance can be improved. As means for attaining the object, a storage battery in which an expand grid is used as a battery plate, the expand grid being a grid member which is formed by expanding a side portion of a collector frame portion  1   a  of a metal sheet  1  to connect a large number of grid wires  1   b  to one another in a net-like shape, is configured so that widths of grid wires  1   b  of a row which is directly connected to the collector frame portion  1   a  of the expand grid, and a lateral end row are larger than widths of grid wires  1   b  of at least one of intermediate rows.

BACKGROUND OF THE INVENTION

The present invention relates to a storage battery in which an expandgrid is used as a battery plate.

A grid which is to be used as a battery plate for a lead storage batteryis sometimes produced by the expansion process. Methods of producing anexpand grid by the expansion process are roughly classified into twokinds or the rotary method and the reciprocal method.

In the reciprocal method, an expand grid is produced in the followingmanner. Dice cutters which are arranged in a stepwise manner arevertically moved on a metal sheet that is intermittently moved, tosequentially form slits in the metal sheet, and the metal sheet is thenstretched to be formed into a net-like shape. Specifically, as shown inFIG. 11, a metal sheet 1 made of lead or a lead alloy is intermittentlytransported on a flat upper face of a lower table 2 in the direction ofthe arrow F. Step-like side faces 2 a are formed on the side faces ofthe lower table 2, respectively. In the step-like side faces 2 a, alarge number of steps (in the figure, only four steps are shown for thesake of simplicity) are formed in such a manner that the distancebetween the side faces is stepwise reduced toward the center by aconstant step difference as advancing in the direction of the arrow F.An upper table 4 to which dice cutters 3 are attached is placed abovethe lower table 2. In practice, the upper table 4 is placed in aposition which is lower in level than the illustrated position, or whichis slightly above the metal sheet 1 transported on the lower table 2,and conducts vertical motions in the position. Step-like side faces 4 awhich are similar to the step-like side faces 2 a of the lower table 2are formed on the side faces of the upper table 4, respectively. Thedice cutters 3 are fixed to the step-like side faces 4 a of the uppertable 4, respectively, with the result that the dice cutters arearranged in a substantially V-like shape as a whole. In each of the dicecutters 3, an edge 3 a is downward projected from the lower face of theupper table 4.

Each time when the intermittent motion is stopped, the upper table 4 islowered to conduct one cycle of the vertical motion, whereby the endportions of the metal sheet 1 are cut and downward stretched by theedges 3 a of the dice cutters 3, resulting in that an expand grid suchas shown in FIG. 12 is formed. Namely, the metal sheet 1 is processedinto an expand grid in which the both side portions of a collector frameportion 1 a in a center area in the width direction are sequentiallystretched into grid wires 1 b that are connected to one another in anet-like shape. The expand grid has a large number of meshes 1 c in theform of meshes each surrounded by four grid wires 1 b. The collectorframe portion 1 a is an area of the metal sheet 1 in which the meshes 1c are not formed in order to enable current collection in a batteryplate, and a plate lug for future connection to a terminal is formed.The expand grid shown in FIG. 12 is produced by an apparatus in which,unlike that shown in FIG. 11, twelve dice cutters 3 are attached to eachof the side faces of the upper table 4.

In the reciprocal production method, the operations of cutting the metalsheet 1 by the dice cutters 3, and stretching and expanding the gridwires 1 b to form the meshes 1 c are completed by one cycle of verticalmotions of the upper table 4. Therefore, the reciprocal productionmethod is conducted so that each of the meshes 1 c is formed into arhombic shape and the four grid wires 1 b surrounding the mesh have thesame length, thereby allowing stress in the process of stretching thegrid wires 1 b to be uniformly applied to the wires. As in the inventiondisclosed in Japanese Patent Publication (Kokai) No. SHO57-90873,another reciprocal production method is known in which each of themeshes 1 c is formed into a substantial parallelogram shape so as tohave long and short edges of different lengths. In the reciprocalproduction method, the grid wires 1 b are stretched straight downward bythe edges 3 a of the dice cutters 3, and hence the grid wires 1 b arenot twisted during the expansion process. Therefore, the method has theadvantage that the grid used as a battery plate of a storage batteryexhibits an excellent life performance.

In the expand grid, the meshes 1 c on an oblique line indicated by, forexample, the one-dot chain line L₁ are formed at one stroke by the dicecutters 3 which are arranged in a substantially V-like shape. When themetal sheet 1 is transported by a predetermined distance as a result ofintermittent motion and the upper table 4 conducts the next verticalmotions, the meshes 1 c on the oblique line indicated by the one-dotchain line L₂ are formed at one stroke. In the metal sheet 1, therefore,the meshes 1 c in the lateral end portions in the width direction arefirst formed, and, each time when the intermittent motion is furtheradvanced, inner meshes 1 c are sequentially formed. The edges 3 a of thedice cutters 3 press down two grid wires 1 b which are arranged in asubstantially V-like shape below the respective meshes 1 c. The gridwires 1 b which are pressed down by the same dice cutter 3 are arrangedin one row along the advancing direction F while being alternatelyinclined in a zigzag manner.

In the thus produced expand grid, as shown in FIG. 12, the grid wires 1b are connected to one another in a net-like shape on both the sides ofthe collector frame portion 1 a which is formed in the center area inthe width direction of the metal sheet 1. When the expand grid is to beused as a battery plate, the collector frame portion 1 a is divided intotwo portions along a cutting line which elongates in the direction ofthe arrow F. In the resulting expand grid which will be used as abattery plate, therefore, the grid wires 1 b in a net-like shape areconnected to one side of the collector frame portion 1 a. In thereciprocal method of producing an expand grid, since the metal sheet 1is intermittently transported, the rate of production is somewhat low.

In the rotary method, an expand grid is produced in the followingmanner. In a slit forming step, first, a large number of zigzag slitsare formed in a metal sheet by using a disk cutter. In an expandingstep, then, the metal sheet is expanded in the width direction tostretch the slits into a net-like shape. Namely, in the rotaryproduction method, an expand grid is produced in the following manner.In the slit forming step shown in FIG. 13, first, the metal sheet 1 ispassed between upper and lower disk cutter rolls 6 each of which isconfigured by a stack of a large number of disk cutters 5, therebyforming slits 1 d. As shown in FIG. 14, each of the disk cutters 5 is ametal disk in which many ridges 5 a and valleys 5 b are alternatelyformed in the peripheral face. In peripheral edge portions of the frontand rear faces of the disk cutter 5, grooves 5 c are formed respectivelyfor the valleys 5 b so as to be opened in the corresponding valleys 5 b.In each of the valleys 5 b, however, the groove 5 c is formed in onlyone of the front and rear faces, and, in the adjacent valleys 5 b, thegrooves 5 c are formed in opposite ones of the front and rear faces,respectively. As shown in FIG. 15, each of the disk cutter rolls 6 isconfigured by stacking a large number of such disk cutters 5 on the sameshaft via spacers 7. The upper and lower disk cutter rolls 6 are placedin positions where the disk cutters 5 are shifted in the axial directionby a half pitch, and the upper and lower peripheral edges arealternately engaged with each other. The upper and lower disk cutterrolls 6 are rotated in synchronization, in opposite directions, and in aphase relationship in which the ridges 5 a and the valleys 5 b of theupper and lower disk cutters 5 are overlapped and engaged with eachother.

When the metal sheet 1 is passed between the disk cutter rolls 6, asshown in FIG. 13, a large number of slits 1 d are formed by the diskcutters 5. In the valleys 5 b of the upper and lower disk cutters 5where the grooves 5 c face each other, the slits 1 d are intermitted,and hence are not continuous along the advancing direction F of themetal sheet 1 but formed with being intermitted at regular intervals.Moreover, the slits 1 d which are formed adjacently in the widthdirection of the metal sheet 1 are shifted by a half pitch in theadvancing direction F, and the slits are formed in a zigzag manner as awhole. The thin metal wire-like portions between the adjacent slits 1 dare formed as grid wires 1 b, and the intermittent portions of the slits1 d along the advancing direction F are formed as nodes 1 e.

Since the grid wires 1 b are pressed in the upward and downwarddirections when the slits 1 d are formed by the ridges 5 a of the upperand lower disk cutters 5, the grid wires 1 b are elastically deformed soas to protrude in the upward and downward directions from the front andrear faces of the metal sheet 1 as shown in FIG. 16(a). All of a seriesof the grid wires 1 b which are arranged via the nodes 1 e along theadvancing direction F are upward pressed by the ridges 5 a of, forexample, the lower disk cutter 5 as shown in FIG. 16(b), whereby thecenter areas are formed as upward protrusions P. All of the series ofthe grid wires 1 b which are adjacent to the grid wires in the advancingdirection F of the metal sheet 1 are downward pressed by the ridges 5 aof the upper disk cutter 5, whereby the center areas are formed asdownward protrusions P.

In the above-described rotary production method, the slits 1 d areformed by passing the metal sheet 1 between the two upper and lower diskcutter rolls 6 which are vertically arranged. Alternatively, the slits 1d may be formed by passing the metal sheet 1 between three or more diskcutter rolls 6.

The metal sheet 1 in which the slits 1 d are formed as described aboveis stretched to both the sides in the width direction to be expanded inan expanding step shown in FIG. 17, to be formed into an expand grid. Ina usual expand grid which is produced by the rotary method, as shown inFIG. 18, the collector frame portion 1 a is disposed in the center areain the width direction of the metal sheet 1, lower frame portions 1 fare disposed in the lateral end portions, respectively, and the largenumber of meshes 1 c in a net-like shape are formed between thecollector frame portion 1 a and the lower frame portions. In thecollector frame portion 1 a and the lower frame portions 1 f, the meshes1 c of the metal sheet 1 are not formed. A plate lug which will beconnected to a terminal for the purpose of current collection is formedon the collector frame portion 1 a. Each of the lower frame portions 1 fis a portion which will function as the lower end of a battery platewhen the plate is placed in a battery case. As shown in FIG. 17, themetal sheet 1 is expanded by further laterally pulling the lower frameportions 1 f in the lateral end portions by expanding apparatuses 8. Theexpanding apparatuses 8 are endless chain apparatuses which are placedso as to form a fan-like shape on respective sides of the transportationpath of the metal sheet 1. Engagement portions attached to chain rollersare engaged with the lower frame portions 1 f of the transported metalsheet 1, so that the metal sheet is stretched obliquely outward.Therefore, the metal sheet 1 is pulled toward the lateral end portionsin the width direction, so that the gaps between the slits 1 d arewidened to be formed into the meshes 1 c of a substantially rhombicshape and the four grid wires 1 b which surround each of the meshes 1 cand which have a substantially same length are connected to one anotherin a net-like shape, thereby producing an expand grid. The grid wires 1b which are formed by the series of adjacent slits 1 d are in the samerow, and arranged in a row along the advancing direction F while beingalternately inclined in a zigzag manner.

When the thus produced expand grid is to be used as a battery plate, thecollector frame portion 1 a which is in the center area in the widthdirection is divided into two portions along a cutting line whichelongates in the direction of the arrow F. In the resulting expand gridwhich will be used as a battery plate, therefore, the grid wires 1 b ina net-like shape are connected to one lateral side of the collectorframe portion 1 a, and the lower frame portion 1 f is in the lateral endportion of the grid.

In the rotary production method, the slit formation and the expansionare conducted while the metal sheet 1 is continuously transported.Therefore, the method has an advantage that the speed of producing anexpand grid can be made larger than that in the reciprocal method.Unlike the case of the reciprocal method in which the cutting andexpansion of the grid wires 1 b are completed at one stroke, however,the grid wires 1 b of the metal sheet 1 suffer two times high stress dueto the slit forming step and the expanding step, because, in the slitforming step, the grid wires 1 b are deformed in either of the upwardand downward directions by the ridges 5 a of the disk cutters 5, and, inthe expanding step, are stretched in order to form the meshes 1 c. Inthe expanding step, moreover, unlike the case of the reciprocal methodin which the grid wires 1 b are pressed only in the downward directionby the dice cutters 3, the grid wires 1 b are stretched via the nodes 1e while being twisted. Therefore, also stress due to the twisting isapplied to the expand grid. Consequently, an expand grid produced by therotary method has a further disadvantage that the grid wires 1 b areeasily ruptured or cracked during the production process to lower theproduction yield and impair the life performance.

In both expand grids which are produced respectively by the reciprocalmethod and the rotary method, usually, the grid wires 1 b have a uniformwidth in every portion. In the case where such an expand grid is used asa positive plate of a lead storage battery, however, an oxidationreaction of lead or a lead alloy occurs during a charging process, andthe reaction proceeds to sometimes cause the grid wires 1 b to becracked by oxidation corrosion. When the grid wires 1 b are cracked, anactive material held in meshes 1 c which are remoter from the collectorframe portion 1 a than the cracked portion is electrically isolated tobe hardly charged and discharged. When the grid wires 1 b in thevicinity of the collector frame portion 1 a are cracked, therefore, alarger quantity of the active material are electrically isolated, withthe result that the capacity of the battery plate is greatly reduced. Tocomply with this, conventionally, a countermeasure is sometimes taken inwhich the grid wires 1 b of rows that are closer to the collector frameportion 1 a are made larger in width. As described above, the influencedue to a crack of corrosion is larger as the cracked row is closer tothe collector frame portion. Namely, techniques such as described belowhave been proposed. In the invention disclosed in Japanese Utility ModelPublication (Kokai) No. SHO61-66864, thicker grid wires are formed in apart of an expand grid. In the invention disclosed in Japanese PatentPublication (Kokai) No. HEI1-204364, the widths of grid wires of anexpand grid are gradually reduced as proceeding from the upper side tothe lower side. In the invention disclosed in Japanese PatentPublication (Kokai) No. HEI9-223502, the thickness of grid wires isdefined.

In a positive plate of a lead storage battery for a communication fieldor the like in which a long life period is particularly requested, anexpand grid is sometimes used in which the metal sheet 1 of a largethickness is used so as to thicken all the grid wires 1 b, whereby thecorrosion resistance is enhanced in order to prevent the grid wires 1 bfrom being cracked by corrosion.

Problem (1) to be Solved by the Invention

In both expand grids produced by the rotary and reciprocal methods, whenthe expand grids are produced so that the grid wires 1 b have differentwidths, grid wires 1 b of a narrower width are easily cracked, therebycausing a problem in that the yield is lowered. This will be describedmore specifically. With respect to an expand grid produced by the rotarymethod, the production process includes the expanding step of stretchingthe metal sheet 1 in the width direction. When the widths of the gridwires 1 b are varied depending on the position in the width direction ofthe metal sheet 1, i.e., the row, therefore, high stress in theexpanding step is applied to a portion of each grid wire 1 b in whichthe grid wire has a small width and hence is low in strength. Moreover,tensile stress produced during the step of stretching the metal sheet 1is first applied to the grid wires 1 b of the rows in the lateral endportions which are remotest from the collector frame portion 1 a. Whenthe widths of such grid wires 1 b of the lateral end rows are smallerthan or equal to those of the grid wires of the other rows,particularly, the grid wires are easily cracked. With respect to anexpand grid produced by the reciprocal method, the meshes 1 c are formedin the sequence from the lateral end portions of the metal sheet 1 whichare remotest from the collector frame portion 1 a in the center area ofthe metal sheet 1, to the inner side. When the grid wires 1 b of a smallwidth and low strength are placed in the lateral end portions,therefore, the grid wires 1 b are easily cracked by vibrations due tothe vertical motions of the upper table 4 or the intermittent motion ofthe metal sheet 1.

Problem (2) to be Solved by the Invention

The rotary production method has the advantages of a high speed ofproducing an expand grid and high productivity, but has a problem inthat, when the metal sheet 1 is made thick in order to enhance thecorrosion resistance, there arises the possibility that the nodes 1 eare ruptured or cracked. Specifically, when the metal sheet 1 is thick,the sectional areas of the grid wires 1 b are inevitably increased. Evenwhen a soft metal such as lead or a lead alloy is used, therefore, therigidity of the grid wires 1 b is enhanced, so that, when the grid wires1 b are stretched in the expanding step, high tensile stress is appliedto the nodes 1 e. The tensile stress is higher as the sectional areas ofthe grid wires 1 b are larger. In the rotary production method in whichthe grid wires 1 b are stretched from both the sides in the expandingstep to be expanded, the tensile stress applied to the nodes 1 e isexcessively high. When a thick metal sheet 1 is used, therefore, anexpand grid is produced by the reciprocal method in most cases. Inpractice, a relatively thin metal sheet 1 is preferably used in therotary production method. Therefore, the rotary production method isoften employed in production of an expand grid to be used as a batteryplate for a lead storage battery for an automobile, and is seldomemployed in production of an expand grid in which a lead sheet having athickness of 1.0 mm or more is used, or an expand grid in which thesectional areas of the grid wires 1 b are 1.0 mm² or more.

When an expand grid in which a thick metal sheet 1 is used is producedby the reciprocal method, there arises a problem in that the activematerial in the battery plate easily drops off during usage.Specifically, when the metal sheet 1 is thick, also the grid wires 1 bbecome thick. In order to increase the capacity density of the batteryplate, therefore, the meshes 1 c into which an active material is to befilled must be enlarged. In an expand grid produced by the reciprocalmethod, the grid wires 1 b are straightly stretched by the dice cutters3, and hence the side faces of the grid wires 1 b are configured bysubstantially flat face, so that the adhesiveness of the active materialis easily impaired. In an expand grid which is produced from a thickmetal sheet 1 by the reciprocal method, therefore, an active materialfilled into large meshes 1 c easily drops off during usage. By contrast,in an expand grid produced by the rotary method, the side faces of thegrid wires 1 b are twisted by the stretching operation during theexpanding step to be formed into curved faces. Therefore, theadhesiveness of the active material is improved, so that, even when anactive material is filled into large meshes 1 c, the active materialdrops off with less probability.

Problem (3) to be Solved by the Invention

The rotary production method has the advantages of a high speed ofproducing an expand grid and high productivity, but has a problem inthat, during the expanding step, high tension acts only on a part of thegrid wires 1 b surrounding each mesh 1 c of a substantially rhombicshape, and hence the grid wires 1 b easily corrode to lower the lifeperformance. In the expanding step in the rotary method, as shown inFIG. 17, the metal sheet 1 is stretched as being transported along theadvancing direction F. As shown in FIG. 19, therefore, higher tension Eacts on grid wires 1 b+ which are more inclined toward the outer side(in FIG. 19, the downward expanding direction) as further proceeding inthe advancing direction F. Namely, each mesh 1 c is made larger duringthe expanding step as further proceeding in the advancing direction F.When each mesh 1 c is deformed in this way, therefore, the grid wires 1b+ which are downward inclined to the right in FIG. 19 receive highertension E to be in a fully stretched condition, but grid wires 1 b−which are downward inclined to the left is to be in a slightly slackenedcondition. When a storage battery is produced by using such an expandgrid which was expanded in an uneven condition of the tension acting onthe grid wires 1 b, as a battery plate, only the grid wires 1 b+ whichreceive the higher tension E corrode at a higher speed, and the lifeperformance of the storage battery is lowered.

SUMMARY OF THE INVENTION

The invention has been conducted in order to solve the above-discussedproblems. It is an object of the invention to provide a storage batteryin which an expand grid is improved with respect to the widths of gridwires, the sectional areas of nodes, and the shapes of meshes, wherebythe productivity of the expand grid can be enhanced and the lifeperformance can be improved.

Means for Solving Problem (1)

The invention set forth in claim 1 provides a storage battery in whichan expand grid is used as a battery plate, the expand grid being a gridmember which is formed by expanding a side portion of a collector frameportion of a metal sheet to connect a large number of grid wires to oneanother in a net-like shape, wherein widths of grid wires of a row whichis directly connected to the collector frame portion of the expand grid,and a lateral end row are larger than widths of grid wires of at leastone of intermediate rows.

According to the invention set forth in claim 1, the grid wires of therow which is directly connected to the collector frame portion are madelarger in width so as to be hardly cracked, and hence it is possible toprevent the capacity of the storage battery from being largely reducedby a crack of corrosion of the grid wires connected to the collectorframe portion. Since also the grid wires of the lateral end row withrespect to the collector frame portion are made larger in width so as tobe hardly cracked, moreover, it is possible to prevent a productionfailure from being caused by a crack of corrosion of the grid wires ofthe end portion during an expanding step in the rotary or reciprocalmethod.

In the expand grid, three or more rows of grid wires are arranged on thelateral side of the collector frame portion. In an expand grid producedby the rotary method, the grid wires of the lateral end row areconnected to another frame portion (lower frame portion) which isdifferent from the collector frame portion. By contrast, in an expandgrid produced by the reciprocal method, the grid is usually terminatedby the grid wires of the lateral end row. A frame portion is anunexpanded area of the metal sheet and having a width which is of acertain degree or sufficiently larger than the widths of the grid wires.A collector frame portion is a frame portion which is used forcollecting currents from grid wires, and on which a plate lug to beconnected to a terminal is usually formed. In the case of the rotarymethod in which a frame portion is formed on each of the sides, one ofthe frame portions is used as the collector frame portion. Grid wireshave a thin strip-like shape which is formed by cutting a metal sheet.As a result of expansion, grid wires in each row are arranged whilebeing alternately inclined in a zigzag manner. A large number of rows ofthe zigzag grid wires are sequentially connected to one another on thelateral side of the collector frame portion, to be formed into anet-like shape. In both the rotary method and the reciprocal method,grid wires of the same row are worked by the same disk or dice cutter,and therefore all grid wires of the same row are usually equal in widthto one another.

The width of a grid wire is not equal to the thickness of the metalsheet itself, but equal to the cutting pitch of the metal sheet by thedisk cutters in the rotary method or the dice cutters in the reciprocalmethod (the width is sometimes reduced as a result of extension of thegrid wire due to expansion). As shown in FIG. 20, when a side portion ofthe collector frame portion 1 a of the metal sheet 1 is expanded by therotary or reciprocal method to be formed into a large number of gridwires 1 b connected in a net-like shape, for example, the width of eachof the grid wires 1 b is not equal to the length Lt depending on thethickness of the metal sheet 1, but equal to the length Lw depending onthe cutting pitch. In FIG. 20, the shapes of the expanded grid wires 1 bare schematically shown in a flat form in order not to differentiatebetween the rotary method and the reciprocal method.

The invention set forth in claim 2 is characterized in that the widthsof the grid wires of the row which is directly connected to thecollector frame portion are largest, and the widths of the grid wires ofthe lateral end row are smaller than the widths of the grid wires of therow which is directly connected to the collector frame portion, andlarger than the widths of the grid wires of at least one of theintermediate rows excluding the row which is directly connected to thecollector frame portion, and the lateral end row.

According to the invention set forth in claim 2, since the grid wires ofthe row which is directly connected to the collector frame portion havethe largest width, a crack of corrosion of the grid wires due tocorrosion during usage which exerts the greatest effect on the lifeperformance can be surely prevented from occurring. Since also the gridwires of the lateral end row with respect to the collector frame portionare made larger in width than those of the intermediate rows so as to behardly cracked, moreover, it is possible to prevent the production yieldfrom being lowered by a production failure.

The invention set forth in claim 3 is characterized in that the largestgrid wire width is not smaller than 1.2 times and not larger than 1.6times the smallest grid wire width.

According to the invention set forth in claim 3, since the largest widthof the grid wires is not smaller than 1.2 times the smallest width, itis possible to surely achieve the effect due to the increased widths ofthe grid wires of the row which is directly connected to the collectorframe portion, and the lateral end row. When the widths of all the gridwires are increased, the rate of the filling quantity of the activematerial with respect to the expand grid in the battery plate isexcessively reduced, and hence the energy density of the storage batteryis lowered. In the invention, therefore, the widths of the grid wires ofthe intermediate rows remain to be as small as possible, so as toprevent the energy density from being lowered. Since the largest widthof the grid wires is not larger than 1.6 times the smallest width, thedifference between the grid wire widths can be prevented from beingexcessively increased to cause weak grid wires of a small width to beparticularly easily cracked during the expanding step in the rotary orreciprocal method.

Means for Solving Problem (2)

The invention set forth in claim 4 provides a storage battery in whichan expand grid is used as a battery plate, the expand grid being a gridmember which is formed by forming a large number of rows of slits thatare intermitted at regular intervals in a metal sheet to arrangeintermittent portions of the slits in a zigzag manner, and stretchingthe metal sheet in a width direction to expand the metal sheet, toconnect grid wires to one another in a net-like shape via nodesconfigured by the intermittent portions, the grid wires being formedbetween slits that are adjacent to each other in the width direction,wherein, in each of the nodes, a node sectional area of a maximumsection along a cutting plane of the slits is two or more times a gridwire sectional area of a section of each of the grid wires, the sectionbeing perpendicular to a longitudinal direction of the grid wire.

According to the invention set forth in claim 4, the node sectional areaof each of the nodes in the expand grid which is produced by the rotarymethod is two or more times the grid wire sectional area of each of thegrid wires. Even when a thick metal sheet is used in order to enhancethe corrosion resistance so that the grid wire sectional area isincreased and high tension acts on the nodes, it is possible to preventthe nodes from being ruptured or cracked.

As shown in FIG. 5 which schematically shows the vicinity of a node ofan expand grid, the node sectional area S_(con) of the node 1 e is thelargest one of sectional areas of sections of the node 1 e along thecutting plane of the slits 1 d (the cutting plane by the disk cutter 5).In FIG. 5, the node 1 e is schematically shown in the form of a regularhexahedron. Actually, the node is pressed by the valleys 5 b of theupper and lower disk cutters 5, and hence has a somewhat stepped shape.Therefore, the largest one of areas of sections of the node 1 e alongthe cutting plane is defined as the node sectional area S_(con). A gridwire sectional area S of the grid wire 1 b is the area of a section ofthe grid wire 1 b which is perpendicular to the longitudinal direction.

The invention set forth in claim 5 is characterized in that each ofmeshes surrounded by the grid wires which are connected to one anotherin a net-like shape via the nodes has an area of 70 mm² or more.

According to the invention set forth in claim 5, even when an expandgrid is produced by using a thick metal sheet in order to enhance thecorrosion resistance and large meshes having an area of 70 mm² or moreare formed, the grid wires are twisted during the expanding step in therotary method. Therefore, the adhesiveness of the active material to themeshes can be enhanced to prevent the active material from dropping offfrom the battery plate.

The invention set forth in claim 6 is characterized in that the gridwire sectional area is not smaller than 1.0 mm² and not larger than 3.5mm².

According to the invention set forth in claim 6, the sectional area ofthe gird wire is 1.0 mm² or more, and, even when tension acting on annode is particularly high, the node sectional area can be set to be 2.0mm² or more. Therefore, it is possible to surely prevent the node frombeing ruptured or cracked. Since the grid wire sectional area neverexceeds 3.5 mm², it is possible to prevent also the overcharge life frombeing shortened.

Means for Solving Problem (3)

The invention set forth in claim 7 provides a storage battery in whichan expand grid is used as a battery plate, the expand grid being a gridmember which is formed by forming a large number of slits in a metalsheet in a zigzag manner, and stretching the metal sheet in a widthdirection to expand the slits into meshes, thereby connecting four gridwires surrounding each of the meshes to one another in a net-like shape,wherein opposed ones of the four grid wires surrounding each of themeshes have a substantially same length, and a length of two opposedones of the grid wires is not smaller than 102% of and not larger than120% of a length of other two opposed ones of the grid wires.

According to the invention set forth in claim 7, since two opposed onesof the grid wires surrounding each of the meshes are longer than theother two opposed grid wires, the mesh has a substantial parallelogramshape having long and short sides of different lengths. When grid wireson the side which receives higher tension during the expanding step inthe rotary method are set to the long sides, therefore, the tensionacting on the grid wires of the long sides is reduced, and the tensionacting on the grid wires of the short sides is increased. Consequently,stress acts in a relatively uniform manner on the four grid wiressurrounding each mesh, so that a phenomenon in which only a part of thegrid wires is easily corroded can be prevented from occurring and thelife performance of the storage battery can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal-section front view showing the case where thethicknesses of disk cutters are varied, taken along the line A-A in FIG.13;

FIG. 2(a) is a plan view of a metal sheet in which slits are formed byusing a disk cutter roll shown in FIG. 1, and FIG. 2(b) is a plan viewof an expand grid which is obtained by expanding the metal sheet;

FIG. 3 is a perspective view schematically showing a step of producingan expand grid in which attachment steps of dice cutters are varied inthe reciprocal method;

FIG. 4 is a plan view of the expand grid which is produced by thereciprocal method shown in FIG. 3;

FIG. 5 is a partial enlarged perspective view of the vicinity of a nodeof an expand grid which is produced by the rotary method;

FIG. 6 is a partial enlarged plan view of an expand grid in which a meshis expanded in an expanding step in the rotary method, into aparallelogram shape having different long and short sides;

FIG. 7 is a view showing changes of the life period and the rate ofcrack of corrosion with respect to the ratio of widths of grid wires;

FIG. 8 is a view showing changes of the number of life cycles withrespect to the grid wire sectional area;

FIG. 9 is a view showing changes of the rate of crack of corrosion withrespect to the node sectional area;

FIG. 10 is a view showing changes of the drop rate of an active materialwith respect to a mesh area;

FIG. 11 is a perspective view schematically showing a production step ofan expand grid by the reciprocal method in a conventional art example;

FIG. 12 is a plan view of the expand grid produced by the reciprocalmethod in the conventional art example;

FIG. 13 is a side view showing a step of forming slits of an expand gridby the rotary method in a conventional art example;

FIG. 14(a) is a side view showing a disk cutter used in the step offorming slits of the expand grid by the rotary method in theconventional art example, FIG. 14(b) is a plan view taken along the lineB-B, and FIG. 14(c) is a partial enlarged side view of the vicinity ofthe line B-B;

FIG. 15 is a longitudinal-section front view showing the conventionalart example, taken along the line A-A in FIG. 13;

FIG. 16(a) is a partial enlarged side view of an expand grid in whichslits are formed in a slit forming step in the rotary method in aconventional art example, and FIG. 16(b) is a partial enlarged planview;

FIG. 17 is a plan view showing a step of expanding an expand grid by therotary method in a conventional art example;

FIG. 18 is a plan view of an expand grid which is produced by the rotarymethod in a conventional art example;

FIG. 19 is a partial enlarged plan view of an expand grid which is inthe course of expansion in an expanding step by the rotary method in aconventional art example; and

FIG. 20 is a partial enlarged perspective view of an expand grid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the invention will be described withreference to the accompanying drawings.

Embodiment (1) of the Invention

FIGS. 1 to 4 show a first embodiment of the invention. FIG. 1 is alongitudinal-section front view showing the case where the thicknessesof disk cutters are varied, taken along the line A-A in FIG. 13, FIG.2(a) is a plan view of a metal sheet in which slits are formed by usinga disk cutter roll shown in FIG. 1, FIG. 2(b) is a plan view of anexpand grid which is obtained by expanding the metal sheet, FIG. 3 is aperspective view schematically showing a step of producing an expandgrid in which attachment steps of dice cutters are varied in thereciprocal method, and FIG. 4 is a plan view of the expand grid which isproduced by the reciprocal method shown in FIG. 3.

In the embodiment, storage batteries in which an expand grid produced byeither of the rotary and reciprocal methods is used as a battery platewill be described.

In the expand grid shown in FIG. 18 and produced by the rotary method,and that shown in FIG. 12 and produced by the reciprocal method, thereduction of the capacity of the battery plate due to a crack ofcorrosion of the grid wires 1 b is greatest when such a crack ofcorrosion occurs in a grid wire 1 b which is close to the collectorframe portion 1 a. Therefore, it is preferable to set the widths of thegrid wires 1 b to be larger so that a crack of corrosion more hardlyoccurs, as the row of the grid wires is closer to the collector frameportion 1 a. On the other hand, in the expand grid shown in FIG. 18 andproduced by the rotary method, higher tensile stress acts during theexpanding step on an end portion which is laterally (in verticaldirection in FIG. 18) remoter from the collector frame portion 1 a, sothat the end portion is easily cracked. In the expand grid shown in FIG.12 and produced by the reciprocal method, the grid wires 1 b of thelateral end portion which is remotest from the collector frame portion 1a, and which is first stretched are easily cracked by process vibrationsdue to the vertical motions of the dice cutters 3 or the intermittentmotion of the metal sheet 1. Therefore, it is preferable to increasealso the widths of the grid wires 1 b which are close to the lateral endwith respect to the collector frame portion 1 a. Usually, a plate lug isformed on the collector frame portion 1 a, and the battery plate isaccommodated in a battery case with directing the plate lug upward. Inthe expand grid, therefore, it is preferable to set the widths of thegrid wires 1 b to be larger as the grid wires are closer to the upper orlower portion. In the most preferable mode of the invention,consequently, the grid wire widths of a row close to the collector frameportion which affect the life performance during usage are set to belargest, the grid wire width is gradually reduced as advancing towardthe laterally intermediate portion, and the grid wire width is graduallyincreased as advancing from the intermediate portion to the lateral end.In the case where only two kinds of grid wires or grid wires of largerand smaller widths are formed, a configuration may be employed in whichonly the grid wires of the one row that is directly connected to thecollector frame portion, or two more rows that are close to thecollector frame portion, and the one row that is in the lateral end withrespect to the collector frame portion, or two more rows that are closeto the lateral end have the larger width, and the grid wires of theintermediate rows other than the rows have the smaller width.

The effect of preventing the grid wires from being cracked cannot besufficiently attained unless the difference between the grid wire widthsis set so that the largest width is at least 1.2 times the smallestwidth. When an extremely large difference is set between the grid wirewidths, however, the strength of grid wires of a larger width isexcessively different from that of grid wires of a smaller width, sothat a production failure often occurs. Therefore, it is preferably toset the largest grid wire width so as not to be larger than 1.6 timesthe smallest grid wire width.

The expand grid of the invention can be obtained in the followingmanner.

In the rotary method, the widths of grid wires of a net-like shape inthe expand grid can be varied depending on the row, by increasing thethickness of a disk cutter which is placed in a portion where the gridwire width is to be increased. When the grid wire width is to bereduced, the thickness of a disk cutter which is placed in thecorresponding portion is reduced.

Steps of forming slits in an expand grid by the rotary production methodwill be described with reference to FIG. 1. As shown in FIG. 13, themetal sheet 1 is passed in the advancing direction F between the diskcutter rolls 6 which are vertically arranged, whereby slits are formedin the metal sheet 1. Also in the case of FIG. 1, in the same manner asthe disk cutter rolls shown in FIG. 15, each of the disk cutter rolls 6is configured by stacking a large number of the disk cutters 5 on thesame shaft via the spacers 7 which are used for forming gaps between thedisk cutters 5. Therefore, the peripheral edges of the disk cutters 5 ofthe upper and lower disk cutter rolls 6 are engaged with each other toform slits in the metal sheet 1 which is inserted between the rolls fromthe front side of FIG. 1. The grid wires 1 b which are configured bygaps between adjacent slits of the metal sheet 1 are pressed by theridges 5 a formed on the circumferential faces of the upper and lowerdisk cutters 5, to be protruded from the lower and upper faces of themetal sheet 1.

In the disk cutter rolls 6, as shown in FIG. 1, the disk cutter 5 andthe spacer 7 which are closest (the left side in the figure) to thecollector frame portion 1 a of the metal sheet 1 have the largestthickness W₁, the spacer 7 and the disk cutter 5 which are next closestto the collector frame portion 1 a, and the spacer 7 and the dick cutter5 which are in the end that is laterally remotest (the right side in thefigure) from the collector frame portion 1 a have the second largestthickness W₂, and the other disk cutters 5 and the other spacers 7 havethe smallest thickness W₃. In the metal sheet 1 in which slits areformed by using the thus configured disk cutter rolls 6, as shown inFIG. 2(a), the width W₁ between the slits which are closest to thecollector frame portion 1 a is largest, the widths W₂ between the slitswhich are next closest to the collector frame portion 1 a, and betweenthose which are remotest from the collector frame portion 1 a are secondlargest, and the widths W₃ between the other slits are smallest. Whenthe metal sheet 1 is expanded, as shown in FIG. 2(b), the width W₁ ofthe grid wires 1 b of the row which is directly connected to thecollector frame portion 1 a is largest, the widths W₂ of the grid wires1 b of the next row which is connected to the above-mentioned grid wires1 b, and the grid wires 1 b of the row which is remotest from thecollector frame portion 1 a are second largest, and the widths W₃ of thegrid wires 1 b of the other rows are smallest. In FIGS. 1 and 2, thedifferences of the widths between the slits, and of the grid wires 1 bare shown in exaggerated form in order to facilitate description. InFIG. 2(b), the widths are shown with neglecting the phenomenon that thewidths of the grid wires 1 b are reduced as a result of extension duringthe expanding process.

In the production method shown in FIGS. 13 and 1, the two upper andlower disk cutter rolls 6 are used. Also in the case where three or moredisk cutter rolls 6 are used, the widths of the grid wires 1 b can bevaried by similarly adjusting the thicknesses of the disk cutters 5 andthe spacers 7.

In the rotary production method described above, the expansion shown inFIG. 2(b) is conducted by, while the collector frame portion 1 a of themetal sheet 1 shown in FIG. 2(a) is fixed, further outward pulling thelower frame portion 1 f in the lateral end portion (the lower endportion in FIG. 2). In the case where the grid wires 1 b which are closeto the lower frame portion 1 f have a small widths W₂ and are low instrength, therefore, there arises the possibility that the grid wires 1b are cracked. This phenomenon is not limited to the grid wires 1 bwhich are close to the lower frame portion 1 f, and occurs in asubstantially similar manner in also the grid wires 1 b which are closeto the collector frame portion 1 a. Also when the pulling direction isreversed, there is the possibility that the grid wires 1 b in thevicinity of the both lateral end portions are cracked during theexpanding process.

In the reciprocal method, the expansion is conducted by vertical motionsof plural dice cutters which are arranged in a stepwise manner above ametal sheet that is intermittently moved. When the step differencebetween two of the dice cutters is increased, therefore, the widths ofgrid wires of the corresponding row can be increased.

An example in which an expand grid is produced by the reciprocal methodwill be described with reference to FIG. 3. In the same manner as thecase of FIG. 11, the metal sheet 1 is intermittently fed in theadvancing direction F. The plurality of step-like side faces 2 a (in thefigure, only four steps are shown) are formed on each of the side facesof the lower table 2 on which the metal sheet 1 is placed andtransported. The upper table 4 is placed above the lower table 2. In thesame manner as the lower table 2, the upper table 4 is provided withplural steps of step-like side faces 4 a on each of the side faces. Thedice cutters 3 each having the edge 3 a for an expanding process areattached to the step-like side faces 4 a of the upper table 4,respectively.

A gap of a size which allows the metal sheet 1 to be passedthrerethrough is formed between the upper table 4 and the lower table 2.The upper table 4 repeats the vertical motions in accordance with theintermittent movement of the metal sheet 1. As a result of the verticalmotions of the upper table 4, the edges 3 a of the dice cutters 3 formslits in the metal sheet 1 and downward stretch the slits to form anet-like grid wires. The step-like side face 2 a which is placed inwardby one step from the largest width portion of the lower table 2corresponds to the lowest stage of the expand grid.

In the embodiment, a larger step difference is disposed in the step-likeside faces 4 a and 2 a of the upper and lower tables 4 and 2.Specifically, referring to FIG. 3, the step difference between thestep-like side face 4 a to which the outermost dice cutter 3 isattached, and the step-like side face 4 a to which the next-inner dicecutter 3 is attached is larger than the other step differences. Thewidths of the grid wires corresponding to the large step difference aremade larger by using the thus configured production apparatus. When sucha step difference is formed in the upper and lower tables 4 and 2 of anactual production apparatus to which the large number of dice cutters 3are attached, the widths W₂ of the grid wires 1 b of an arbitrary rowcan be made larger than the widths W₃ of the grid wires 1 b of the otherrows as shown in, for example, FIG. 4. By adequately selecting the rowsin which the widths of the grid wires 1 b are to be enlarged, in thesame manner as the rotary method shown in FIG. 2(b), it is possible toincrease only the widths of the grid wires 1 b of the rows which areclose to the collector frame portion 1 a, and the row which is remotefrom the collector frame portion.

Embodiment (2) of the Invention

FIG. 5 shows a second embodiment of the invention, and is a partialenlarged perspective view of the vicinity of a node of an expand gridwhich is produced by the rotary method.

In the embodiment, a storage battery in which an expand grid produced bythe rotary method is used as a battery plate will be described.

FIG. 5 is an enlarged schematic view showing the vicinity of the node 1e of the expand grid which is produced by the rotary method. In theexpand grid of the embodiment, the node sectional area S_(con) of thenode 1 e which is the largest one of sectional areas of sections of thenode 1 e along the cutting plane of the slits 1 d is set to be two ormore times the grid wire sectional area S of a section of the grid wire1 b which section is perpendicular to the longitudinal direction.According to the configuration, it is possible to prevent the node 1 efrom being deformed, ruptured, or cracked by tension which is producedwhen the grid wires 1 b are stretched to both the sides in the expandingstep in the rotary method. Even when meshes having an area of 70 mm² ormore are formed, the grid wires 1 b are twisted during the process ofstretching to both the sides, in the expanding step in the rotarymethod. Therefore, the adhesiveness of the active material is higherthan that in an expand grid which is produced by the reciprocal method,and in which the side faces of the grid wires 1 b are formed as flatfaces, and hence can be prevented from dropping off from the batteryplate. When the areas of the meshes are excessively enlarged, however, alarge quantity of the active material drop off during a production stepin a mass production process. Therefore, it is preferable to set themesh area so as not to be larger than 150 mm².

In an expand grid which is produced by the rotary method in theconventional art, deformation, a rupture, a crack of corrosion, or alike failure occurs very easily in the vicinities of connecting areas ofthe nodes 1 e with the grid wires 1 b. When the grid wire sectional areaS of each grid wire 1 b is set not to be smaller than 1.0 mm², such afailure can be surely prevented from occurring. In consideration of theovercharge life, however, it is preferable to set the grid wiresectional area S so as not to be larger than 3.5 mm².

Embodiment (3) of the Invention

FIG. 6 shows a third embodiment of the invention, and is a partialenlarged plan view of an expand grid in which a mesh is expanded in anexpanding step in the rotary method, into a parallelogram shape havingdifferent long and short sides.

In the embodiment also, a storage battery in which an expand gridproduced by the rotary method is used as a battery plate will bedescribed.

As shown in FIG. 6, the meshes 1 c of the expand grid which is producedby the rotary method have a substantial parallelogram shape, exceptthose which are adjacent to the collector frame portion 1 a, and whichhave a substantially triangular shape. Namely, each of the meshes 1 c isconfigured by the four nodes 1 e which are positioned respectively atthe apexes of the parallelogram, and the four grid wires 1 b whichconnect the nodes 1 e to one another.

As shown in FIG. 18, each of the meshes 1 c of the conventional expandgrid which is produced by the rotary method has a substantialparallelogram shape having the sides of the same length, or asubstantially rhombic shape. Therefore, the four grid wires 1 bsurrounding the mesh 1 c are approximately equal in length to oneanother. By contrast, in the embodiment, as shown in FIG. 6, each of themeshes 1 c has a substantial parallelogram shape having long and shortsides of different lengths. In the four grid wires 1 b surrounding themesh 1 c, the two grid wires 1 b+which are more inclined toward theouter side (in FIG. 6, the downward expanding direction) as proceedingin the advancing direction F of the metal sheet 1 are made longer, andthe two grid wires 1 b− which are inclined toward the inner side (inFIG. 6, toward the collector frame portion 1 a in the upper side) asproceeding in the advancing direction F are made shorter. The longergrid wires 1 b+ are formed so as to have a length which is not smallerthan 102% of and not larger than 120% of the length of the shorter gridwires 1 b−.

The expand grid in which the grid wires 1 b surrounding each mesh 1 chave different lengths can be produced by the rotary method with settingevery second ridge 5 a of the disk cutter 5 shown in, for example, FIG.14 to have a larger amount of protrusion toward the outer peripheralside (the peripheral length along the protrusion shape of the ridge 5 ais increased). When the slits 1 d are formed by the conventional diskcutters 5 in which the ridges 5 a protrude in the same amount, the gridwires 1 b which are pressed by the ridges 5 a to be protrudinglydeformed in the upward and downward directions have the same length asshown in FIG. 16(a). By contrast, the grid wires 1 b which are pressedby the ridges 5 a of the larger protrusion amount are largelyprotrudingly deformed in the upward and downward directions, andstretched to be prolonged as a result of the deformation. When the metalsheet 1 which has undergone the slit forming step in this way isexpanded in the expanding step, the meshes 1 c surrounded by the gridwires 1 b of different lengths are formed into a substantialparallelogram shape having long and short sides of different lengths asshown in FIG. 6. In this case, the longer grid wires 1 b+ are set so asto be more inclined toward the outer side as further proceeding in theadvancing direction F of the metal sheet 1.

In the thus configured expand grid, although the longer grid wires 1 b+are largely deformed in the slit forming step in the rotary method, thelonger grid wires can avoid receiving high tension in the expanding stepbecause the longer grid wires have been already largely stretched. As aresult, it is possible to cause the two longer grid wires 1 b+ and thetwo shorter grid wires 1 b− which surround each mesh 1 c to receivesubstantially equal stress in the slit forming and expanding steps.

Expand grids of various ratios of the length of the longer grid wires 1b+ to that of the shorter grid wires 1 b− have been produced andstudied. As a result, it has been found that, when the length ratio isnot smaller than 102% and not larger than 120%, all the grid wires 1 bare effectively prevented from being corroded. This is because of thefollowing reason. When the length ratio is smaller than 102%, there isno substantial difference as compared with a conventional expand gridproduced by the rotary method, or corrosion is caused to proceed only inthe longer grid wires 1 b+ by the unbalance of tension in the expandingstep, and, when the length ratio is larger than 120%, the expand grid isnot sufficiently expanded in the expanding step to produce a state inwhich the longer grid wires 1 b+ are slackened. When the length of thelonger grid wires 1 b+ is not smaller than 106% and not larger than115%, moreover, the difference between the progress of corrosion in thelonger grid wires 1 b+, and that in the shorter grid wires 1 b− can bemade very small.

EXAMPLE 1

Example 1 is an example of the first embodiment, and shows comparisonbetween the case where the grid wires 1 b which are directly connectedto the lower frame portion 1 f that is remotest from the collector frameportion 1 a of the metal sheet 1 shown in FIG. 2 were set to have thesmallest width, and that where the grid wires were set to have a largerwidth.

In Example 1, an alloy sheet of Pb-0.06 wt. % Ca-1.0 wt. % Sn wasprocessed into expand grids by using the rotary production method. Whenthe grid wires 1 b of the row which is directly connected to the lowerframe portion 1 f that is remotest from the collector frame portion 1 awere set to have the smallest width by adequately changing thethicknesses of disk cutters and spacers of a rotary expanding machine, acrack of corrosion was observed in 10% of the connections of the gridwires 1 b and the lower frame portion 1 f, during the expanding step. Bycontrast, when the grid wires 1 b of the row which is directly connectedto the lower frame portion 1 f were set to be larger in width than thegrid wires 1 b of the other or intermediate rows, the rate of crack ofcorrosion in the connections of the grid wires 1 b and the lower frameportion 1 f during the expanding step was reduced to about 2%.

Similar tests were conducted by using the reciprocal production method.In this case also, in the same manner as the case of the rotaryproduction method, when the grid wires of the row which is remotest fromthe collector frame portion 1 were set to have the smallest width, acrack of corrosion was observed in about 6% of the grid wires during theexpanding step. By contrast, when the widths of the grid wires wereincreased, the rate of crack of corrosion during the expanding step wasreduced to about 1%.

EXAMPLE 2

Example 2 is an example of the first embodiment, and shows comparisonbetween the case where the grid wires 1 b which are directly connectedto the collector frame portion 1 a of the metal sheet 1 shown in FIG. 2were set to have the smallest width, and that where the grid wires wereset to have a larger width.

In Example 2 also, an alloy sheet of Pb-0.6 wt. % Ca-1.0 wt. % Sn wasprocessed into expand grids by using the rotary production method. Ananode active material for a lead storage battery which was produced inthe usual way was filled into the expand grids produced by the rotaryproduction method, and the grids were then cured and dried to formpositive plates for a lead storage battery. A cathode active materialfor a lead storage battery which was produced in the usual way wasfilled into identical expand grids, and the grids were then cured anddried to form negative plates for a lead storage battery.

Such positive and negative plates were alternately stacked viaseparators which are configured mainly by microporous polyethylene, andthe plates of the same polarity were then connected to one another toform a plate group. Such plate groups were inserted into a battery case,and a given amount of dilute sulfuric acid electrolyte of apredetermined specific gravity was poured into the case, therebyproducing lead storage batteries of Type 75D26 according to JIS.

In the example also, expand grids of various kinds of grid wire widthswere produced by changing the thicknesses of disk cutters and spacers ofa rotary expanding machine.

The lead storage batteries of Type 75D26 were subjected to a test ofovercharging a battery at 75° C. (in which one cycle is set as thecharging voltage of 13.8 V (the limit current of 25 A), the chargingtime of 117 h, the stand time of 49 h, and the discharging current of200 A, and which is ended when the 2 second voltage at discharge becomes3.0 V or lower).

When the grid wires 1 b of the row which is directly connected to thecollector frame portion 1 a were set to have the smallest width, it wasearly judged that the battery life is exhausted. Lead storage batteriesin which the life is exhausted were disassembled, and the conditions ofthe positive plates were observed. As a result, the collector frameportion 1 a and the grid wires 1 b which are directly connected to theportion were largely corroded, and, in a most severely corroded plate,the grid wires 1 b were completely separated from the collector frameportion 1 a.

When the grid wires 1 b of the row which is directly connected to thecollector frame portion 1 a were set to have a larger width, theabove-mentioned phenomenon did not occur, and the life performance wasmore excellent as the grid wires 1 b have a larger width. When also thewidths of the grid wires 1 b of the row adjacent to that which isdirectly connected to the collector frame portion 1 a were set to belarger, the life performance was further improved.

Tests similar to those described above were conducted on grids producedby using the reciprocal production method. The results of the tests wereidentical with the test results in the case of the rotary productionmethod.

EXAMPLE 3

Example 3 is an example of the first embodiment, and shows comparison ofexpand grids in which the ratio of the width of the widest grid wire tothat of the narrowest grid wire was variously changed.

In Example 3 also, an alloy sheet of Pb-0.6 wt. % Ca-1.0 wt. % Sn wasprocessed into expand grids by using the rotary production method. Ananode active material for a lead storage battery which was produced inthe usual way was filled into expand grids produced by the rotaryproduction method, and the grids were then cured and dried to formpositive plates for a lead storage battery. A cathode active materialfor a lead storage battery which was produced in the usual way wasfilled into identical expand grids, and the grids were then cured anddried to form negative plates for a lead storage battery.

Such positive and negative plates were alternately stacked viaseparators which are configured mainly by microporous polyethylene, andthe plates of the same polarity were then connected to one another toform a plate group. Such plate groups were inserted into a battery case,and a given amount of dilute sulfuric acid electrolyte of apredetermined specific gravity was poured into the case, therebyproducing lead storage batteries of Type 75D26 according to JIS.

In the example also, expand grids of various kinds of grid wire widthswere produced by changing the thicknesses of disk cutters and spacers ofa rotary expanding machine.

The lead storage batteries of Type 75D26 were subjected to a test ofovercharging a battery at 75° C. (in which one cycle is set as thecharging voltage of 13.8 V (the limit current of 25 A), the chargingtime of 117 h, the stand time of 49 h, and the discharging current of200 A, and which is ended when the voltage at 2 seconds becomes 3.0 V orlower). In the expand grids, grid wires of two rows which are close tothe collector frame portion have the largest width, those of two rowswhich are next closest to the collector frame portion have the secondlargest width, and those of two rows which are remotest from thecollector frame portion and close to the lower frame portion have thesmallest width.

FIG. 7 shows results of the tests. In FIG. 7, the abscissa indicates theratio of the width of the widest grid wire to that of the narrowest gridwire, and the ordinate indicates a life performance ratio with respectto the life performance of a lead storage battery using expand grids inwhich all grid wires have the same width, the life performance being setto 100, and the rate of crack of corrosion during production of anexpand grid. As apparent from FIG. 7, in lead storage batteries usingexpand grids of the configuration of Example 3, the life performance wasmore remarkably improved as widths of grid wires are larger, but, whenthe width ratio of grid wires was made larger than 1.6 times, also therate of crack of corrosion during production of an expand grid wasabruptly increased. This was caused by a phenomenon that, during thestep of expanding an expand grid, a grid wire of a larger width ishardly stretched, that of a smaller width is more easily stretched, andstress produced in the expanding step is therefore concentrated to gridwires of a smaller width.

Tests similar to those described above were conducted on grids producedby using the reciprocal production method. The results of the tests wereidentical with the test results in the case of the rotary productionmethod.

Also in storage batteries which are other than a lead storage battery,and in which anode current collectors are corroded, similar results wereobtained.

EXAMPLE 4

Example 4 is an example of the second embodiment. In the example, expandgrids produced by the rotary method were investigated for overchargelife while changing the grid wire sectional area S of the grid wires 1 bof each expand grid.

In a method of producing an expand grid by using the rotary method, thethicknesses of the grid wires 1 b can be varied by changing thethickness of the metal sheet 1, and the widths of the grid wires 1 b canbe varied by changing the thickness of the disk cutter 5 used in theslit forming step. Plural sets of the disk cutters 5 in differentthicknesses were produced. While adequately replacing the disk cutterset with another one, and changing the thickness of the metal sheet 1,expand grids in which the grid wire sectional area S of the grid wires 1b ranges from 0.64 mm² (thickness: 0.8 mm×width: 0.8 mm) to 4.0 mm²(thickness: 2.0 mm×width: 2.0 mm) were produced by the rotary method.The expand grids are identical with one another in weight and externaldimensions. After an active material was filled into the expand grids,the expand grids were cured and dried to form positive plates.

The positive plates of different grid wire sectional areas S, andnegative plates which were produced by a conventional method werecombined with separators which are configured mainly by microporouspolyethylene to produce lead storage batteries of Type 55D23 (JapaneseIndustrial Standard JIS D 5301) for an automobile. A given amount ofdilute sulfuric acid of a predetermined specific gravity was poured andformation was performed to complete the lead storage batteries. Anovercharge life test (according to the test method of JIS D 5301) wasconducted on the lead storage batteries.

FIG. 8 shows relationships between the grid wire sectional area S andthe number of life cycles in the overcharge life test. As shown in FIG.8, when the grid wire sectional area S of the grid wires 1 b is smallerthan 1.0 mm², the battery life is early exhausted due to corrosion ofthe grid, but, when the grid wire sectional area S is not smaller than1.0 mm², the number of life cycles apparently shows a tendency to befurther increased as the sectional area is larger. When the grid wiresectional area S is larger than 3.0 mm², the mesh size becomes large,and hence the active material is softened or drops off, so that thenumber of life cycles begins to be reduced. Therefore, it is preferableto set the grid wire sectional area S so as not to be larger than 3.5mm².

EXAMPLE 5

Example 5 is an example of the second embodiment. In the example, expandgrids produced by the rotary method and having different grid wiresectional areas S of the grid wires 1 b were investigated for rate ofcrack of corrosion, while changing the node sectional area S_(con) ofthe nodes 1 e.

In a method of producing an expand grid by using the rotary method, thelength of each node 1 e in the direction of the slits 1 d can be changedby changing the widths of the valleys 5 b and the grooves 5 c of thedisk cutters 5 used in the slit forming step. Therefore, plural sets ofthe disk cutters 5 of different widths of the valleys 5 b and thegrooves 5 c were produced, and expand grids were then produced by therotary method while adequately replacing the disk cutter set withanother one, and changing the grid wire sectional area S of the gridwires 1 b by the method of Example 4. In this case, for the sake ofcomparison, the ridges 4 a of the disk cutters 5 were always maintainedconstant in length on a circumference so that the longitudinal lengthsof the grid wires 1 b are equal to one another.

The manner of changes of the rate of crack of corrosion was investigatedwhile changing the node sectional area S_(con) for each of several kindsof different grid wire sectional areas S. FIG. 9 shows results of theinvestigation for the relationship between the ratio of the nodesectional area S_(con) to the grid wire sectional area S, and the rateof crack of corrosion with using various grid wire sectional areas S asparameters. As apparent from the figure, for any grid wire sectionalarea S, when the node sectional area S_(con) is not smaller than twotimes (2S) the grid wire sectional area, the rate of crack of corrosionis surely reduced. When the grid wire sectional area S is not smallerthan 1.0 mm², the reduction of the rate of crack of corrosion in thecase where the node sectional area S_(con) is not smaller than two times(2S) becomes remarkable.

EXAMPLE 6

Example 6 is an example of the second embodiment. In the example, thedrop rate of an active material which was filled into expand gridsproduced by the reciprocal and rotary methods was investigated.

Sets of plural expand grids were produced by the reciprocal and rotarymethods so as to be equal to one another in weight and grid wiresectional area S. In the expand grids, each mesh has an area in therange of 50 mm² to 225 mm². After an active material was filled into theexpand grids, the expand grids were cured and dried to form positiveplates. The positive plates and negative plates which were produced by aconventional method were combined with separators which are configuredmainly by microporous polyethylene to produce lead storage batteries ofType 55D23 (Japanese Industrial Standard JIS D 5301) for an automobile.A given amount of dilute sulfuric acid of a predetermined specificgravity was poured and formation was performed to complete thebatteries. A light-load life test (according to the test method of JIS D5301) was conducted on the lead storage batteries. The life test wasinterrupted at 3,000 cycles. The plates were taken out and horizontallyplaced. Thereafter, the drop rate (the number of dropping meshes/thetotal number of meshes) of the active material was investigated.

FIG. 10 shows results of the investigation for the relationship betweenthe mesh area and the drop rate of an active material in both thereciprocal and rotary methods. It was observed that, in expand gridsproduced by the reciprocal method, the active material dropped at ahigher rate when the mesh area was not smaller than 70 mm². By contrast,it was proved that, in expand grids produced by the rotary method, thedrop rate is about a half of that in expand grids produced by thereciprocal method, or the drop rate of an active material is remarkablyreduced.

EXAMPLE 7

Example 7 is an example of the third embodiment. In the example, theratio of corrosion amounts of two kinds of grid wires 1 b in the casewhere the longer grid wires 1 b+ among the grid wires surrounding eachmesh 1 c in an expand grid was changed from 100% to 130% wasinvestigated.

Plural sets of disk cutters in which all the ridges 5 a of the diskcutters 5 shown in FIG. 14 have the same amount of protrusion, and thosein which every second ridge 5 a of the disk cutters 5 to have a largeramount of protrusion were produced. Disk cutter rolls 6 were assembledby using such disk cutter sets, respectively. The slits 1 d were formedin the metal sheet 1 in the slit forming step by using each of the diskcutter rolls 6. The metal sheet was expanded in the expanding step toproduce an expand grid. As a result, expand grids were produced inwhich, among the grid wires surrounding each mesh 1 c, the length of thetwo longer grid wires 1 b+ ranges from 100% to 130% of that of the twoshorter grid wires 1 b−. All the expand grids have the same dimensionsof the length of 115 mm, the width of 137 mm, and the thickness of 0.9mm. The meshes 1 c were adjusted in size so that the weight of oneexpand grid is substantially constant even when the shapes of the meshes1 c are varied.

An active material was filled into all of the expand grids having themeshes 1 c of different shapes. Thereafter, the grids were cured anddried to form positive plates. The positive plates, and negative plateswhich were produced by a conventional method were combined withseparators which are configured mainly by microporous polyethylene toproduce lead storage batteries of Type 55D23 (Japanese IndustrialStandard JIS D 5301) for an automobile. A given amount of dilutesulfuric acid of a predetermined specific gravity was poured andformation was performed to complete the lead storage batteries. The leadstorage batteries were subjected to an overcharge test in which thebatteries were placed in a water tank of 60° C. and then charged by 4.8A for 30 days. After the overcharge test was completed, the lead storagebatteries were disassembled, and the positive plates were taken out. Thepositive plates were rinsed well under running water, and then dried ina gaseous phase of 50° C. for 48 hours. Thereafter, the positive plateswere impregnated with an epoxy resin. The epoxy resin was cut, the cutsurface was mirror-polished, and the size of the grid was measured undera stereoscopic microscope to obtain the amount of corrosion.

Table 1 below shows results of the tests. TABLE 1 Length rate of longergrid Ratio of amounts of corrosion wire to shorter grid (longer gridwire:shorter wire (%) grid wire) Remarks 100% 65:35 Conventional example102% 58:42 Example 106% 54:46 Example 110% 51:49 Example 115% 43:57Example 120% 40:60 Example 130% — Failure

As seen from Table 1 above, when the grid wires 1 b− and 1 b+ have thesame length (the length ratio is 100%), the amounts of corrosion are65:35 or the larger amount of corrosion is about two times the smallerone. By contrast, when the grid wires 1 b+ are prolonged to attain theratio of from 102% to 120%, the difference between the amounts ofcorrosion is reduced while the difference in the case of 110% isminimum. When corrosions of the grid wires 1 b+ and 1 b− proceed indifferent rates, the grid wires 1 b which are precedently corroded areearly subjected to a crack of corrosion. By contrast, in the case wherethere is no difference between the amounts of corrosion, corrosionsproceed in a substantially same manner, and hence the longest life canbe attained. In order to sufficiently enhance the lengthening of life,therefore, it is further preferable to set the length of the grid wires1 b+ to be in the range of from 106% to 115% so that the differencebetween the amounts of corrosion shown in Table 1 can be suppressed to15% or less. In the case where the length of the grid wires 1 b+ is 130%or longer, however, an expand grid was not sufficiently expanded in theexpanding step to produce a state in which the longer grid wires 1 b+are slackened, and hence the thickness of the grid was larger than apredetermined value. Therefore, such an expand grid was not subjected tothe overcharge test.

In addition to the examples, similar tests were conducted while changingthe dimensions of an expand grid, the size of the meshes 1 c, the alloycomposition of the metal sheet 1, the gravity of the electrolyte, etc.In all the tests, results which are substantially identical with thoseof the examples were obtained.

As apparent from the above description, according to the storage batteryof the first embodiment of the invention, the widths of the grid wiresof the expand grid are varied, whereby the corrosion resistance andproductivity of a battery plate using the expand grid can be improved,so that the life performance of the storage battery can be enhanced.

According to the second embodiment, even in an expand grid which isproduced from a thick metal sheet by the rotary method, nodes can beprevented from suffering a crack of corrosion by increasing thesectional area of the nodes. The expand grid is produced by the rotarymethod. Even when a thick metal sheet is used and the meshes areenlarged, therefore, an active material can be prevented from droppingoff.

According to the third embodiment, the lengths of the four grid wiressurrounding each mesh in an expand grid which is produced by the rotarymethod are varied, whereby corrosions are caused to proceed in asubstantially uniform manner, so that the life performance of a storagebattery in which the expand grid is used as a battery plate can beimproved.

1. A method for preparing a storage battery comprising: expanding ametal sheet to form an expand grid comprising a grid member and acollector frame portion, said grid member being connected to saidcollector frame portion and comprising grid wires connected to oneanother in a net shape via nodes; forming an electrode plate comprisingsaid expand grid; and integrating said electrode plate in said storagebattery, wherein said grid member comprises a first row of grid wireswhich is nearest to said collector frame portion, a second row of gridwires which is farthest from said collector frame portion andintermediate rows of grid wires between said first row and said secondrow; and grid wires of said first row and said second row are of greaterwidths than grid wires of at least one of said intermediate rows.
 2. Amethod for preparing a storage battery according to claim 1, wherein thegrid wires of said first row are the grid wires of greatest width insaid grid member and are of greater width than grid wires of said secondrow.
 3. A method for preparing a storage battery according to claim 2,wherein the greatest width of grid wires in said grid member is notsmaller than 1.2 times and not larger than 1.6 times the smallest widthof grid wires in said grid member.
 4. A method for preparing a storagebattery according to claim 3, wherein all grid wires of a same row insaid grid member are substantially equal in width and said same row hasa longitudinal direction which is substantially parallel to alongitudinal direction of said collector frame portion.
 5. A method forpreparing a storage battery according to claim 2, wherein all grid wiresof a same row in said grid member are substantially equal in width andsaid same row has a longitudinal direction which is substantiallyparallel to a longitudinal direction of said collector frame portion. 6.A method for preparing a storage battery according to claim 1, whereinthe greatest width of grid wires in said grid member is not smaller than1.2 times and not larger than 1.6 times as large as the smallest widthof grid wires in said grid member.
 7. A method for preparing a storagebattery according to claim 6, wherein all grid wires of a same row insaid grid member are substantially equal in width and said same row hasa longitudinal direction which is substantially parallel to alongitudinal direction of said collector frame portion.
 8. A method forpreparing a storage battery according to claim 1, wherein all grid wiresof a same row in said grid member are substantially equal in width andsaid same row has a longitudinal direction which is substantiallyparallel to a longitudinal direction of said collector frame portion.